Study Shows Carbonates In Martian Meteorite Are 3.9 Billion Years Old

MARTIAN METEORITE CARBONATES 3.9 BILLION YEARS OLD

A new study of the carbonate minerals found in a meteorite from Mars
shows they were formed about 3.9 billion years ago. Scientists believe
the planet had flowing surface water and warmer temperatures then,
making it more Earth-like. Giant meteorites were blasting huge craters
in its surface.

This study doesn't directly address the possibility that life once
existed on Mars. But "It's another piece in the puzzle," said Larry E.
Nyquist of the Planetary Sciences Branch of Johnson Space Center's Earth
Science and Solar System Exploration Division. Nyquist, one of the
authors of an article in Science, a weekly publication of the American
Association for the Advancement of Science, was the principal
investigator.

Researchers at Johnson Space Center in Houston and the University of
Texas at Austin did the study, using different techniques. Both produced
similar results, establishing the carbonates' age within comparatively
narrow limits.

The 4.2 pound meteorite is believed to be part of an igneous rock
formation formed about 4.5 billion years ago as Mars solidified from a
molten mass. The meteorite probably was blasted from the planet when a
huge comet or asteroid struck Mars 16 million years ago.

It fell in Antarctica about 13,000 years ago, and was found in 1984 by
an annual expedition sponsored jointly by NASA, the National Science
Foundation, and the Smithsonian Institution. Called ALH84001, after the
Allan Hills in Antarctica where it was found, it was returned to Johnson
Space Center, and has been preserved at the Meteorite Processing
Laboratory there.

It subsequently was recognized as one of more than a dozen meteorites
with unique Martian characteristics.

Just how the carbonates were deposited within this igneous rock is the
topic of lively debate. Some scientists believe water saturated with
carbon dioxide from the atmosphere seeped down to the subsurface site
where the igneous rock formed and created the carbonate deposits. On
Earth, living organisms often play a role in carbonate formation. In
1996 scientists at Johnson Space Center and Stanford University examined
the carbonates in ALH84001 using electron microscopy and laser mass
spectrometry, and reported evidence suggesting primitive life may have
existed in them.

Other scientists believe the carbonates formed when hot,
carbon-dioxide-bearing fluids were forced into cracks in the rocks when
a meteor struck Mars. The 3.9-billion-year age of the carbonates
eliminates neither possibility.

The carbonates themselves are tiny deposits, reddish globules, some with
purplish centers and many surrounded by white borders. The different
colors are due to variations in the compositions of the carbonates:
purplish manganese-bearing calcium carbonate, reddish iron carbonate,
and white magnesium carbonates. The globules were found along fractures
in the meteorite and make up about 1 percent of its volume.

The JSC-UT team, using a binocular microscope and tools resembling
dental picks, over a period of months painstakingly separated out enough
of the carbonate material for their analyses. After experimenting with
terrestrial calcium, iron, and magnesium carbonates, they developed a
way to selectively dissolve carbonate material of differing
compositions, enabling them to separate different elements from the
carbonate solutions.

The study established the age of the carbonate deposits by measuring the
decay of rubidium to strontium and of uranium to lead. The techniques
are similar to carbon dating, which is used for much shorter time
periods. The investigators used the dual approach because "we wanted to
make sure we had a result we could believe in and that other people
could believe in," Nyquist said.

The leading author of the Science article is Lars E. Borg, formerly of
the National Research Council and Johnson Space Center and now at the
University of New Mexico in Albuquerque. Other authors are James N.
Connelly of the University of Texas at Austin, Chi-Yu Shih, Henry
Weismann, and Young Reese, of Lockheed Engineering and Science in
Houston. K. Manser of the University of Texas contributed to the
investigation.

The age of the carbonates, said Everett K. Gibson of Johnson Space
Center and an author of the 1996 study that reported evidence of
microbial life in the carbonates, had been "one of the real mysteries"
of indications of life on Mars. Had the carbonates been formed more
recently, when the planet's surface was devoid of water, it would have
been unlikely they were associated with primitive life on Mars. Dating
them at 3.9 billion years, when there apparently was surface water on
Mars is, Gibson said, very important, and could "suggest events were
very similar in the inner solar system" as primitive life arose.